Air Pollution in Subway Systems May Be Much Worse Than We Realized

April 26, 2017

As anyone who has ridden the subway knows, the air down there is unpleasant. New research done in Canada shows that air pollution levels in Toronto’s subway system are ten times greater than those above ground. It’s a troubling realization for subway-goers, but there are ways to keep these underground systems clean.

A new study led by University of Toronto chemical engineer Greg Evans and published in Environmental Science and Technology shows that trains and platforms along the Toronto Transit Commission (TTC) have the highest levels of air pollution in any of Canada’s three subway systems—10 times higher than outside air. Pollution levels in Toronto’s subway were three times worse than the air quality in Montreal’s Metro, while Vancouver was rated the cleanest of the nation’s three subway systems.

To obtain these results, the researchers recruited university students and equipped them with portable instruments that measured tiny airborne particles smaller than 0.00025 centimeters, or 25 micrograms. Particulate matter this small is easily inhaled, and can cause breathing problems and damage to lung tissue.

Pollution in Toronto is typically around 10 micrograms per cubic meter of air, but it can hit 30 on a bad day. Along the TTC, however, levels reached as high as 100 micrograms of pollutants per cubic meter. That’s as bad as Beijing on a typical day. A follow-up analysis showed high concentrations of metals, which offer a clue to the source of the pollution: the wheels and brakes on the subway trains themselves.

“When you’re standing on the platform, you can sense the blast of air as the subway comes in,” explained Evans to Gizmodo. “That’s because the train is moving down the tunnel like a piston, pushing the air in front of it. Subways are primarily underground, so there’s nowhere for the particles to go. When the train comes into the station, it causes dust and particles to become airborne.” Evans thinks much of the particulate matter is coming from the metal wheels on the rails. Traces of barium were also found in the air samples, which are likely coming from the brakes themselves.